Contractors working under the LH Ford bridge are growing increasingly nervous after becoming the target of thrown items, including a children's scooter.
In the past few weeks there have been three incidents of workers getting hit by rocks.
Roads and Maritime Services director western region Alistair Lunn said rocks, stones, ice and a children's scooter had been thrown from the top of the bridge deck to the 15 metres below where the workers are.
"They know the workers are there because there's laughing and it's seen as a bit of entertainment, but it's got some really serious consequences around the safety and wellbeing of those workers down there," Mr Lunn said.
"It has other affects because they can't focus on their work because they're worried about what else might be happening, therefore that could lead to an accident on the work site because they're not focusing."
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Work is currently being undertaken to strengthen the LH Ford bridge. The $10 million project is expected to take another eight or nine months to complete.
Mr Lunn said it was school-aged children who had been seen throwing the items. As well as working with the police and local schools, the RMS has turned to more serious measures.
"We've got in place some CCTV and we're also looking at engaging a security guard now to work on the bridge deck for us while we've got workers down there, which obviously comes at a cost to the taxpayer. And it's a really unfortunate outcome," he said.
The rock-throwing started six or seven weeks ago and after a campaign on social media it seemed to peter out, but Mr Lunn said it had started up again.
He said the workers were "frustrated" and "nervous" about the incidents but understood it was a small minority taking part.
"This is a contractor that's come to Dubbo to do this really important work, strengthening the LH Ford Bridge and they're obviously seeing a few individuals not behaving in the right manner," Mr Lunn said.
Mr Lunn has implored for anyone who sees the kind of behaviour to report it to the Dubbo Police or call the RMS' project hotline, which is on the signs near the bridge.
The director said those throwing the items needed to realise it wasn't acceptable behaviour.
"I'm sure their parents wouldn't think it's entertainment, " Mr Lunn said